1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to a system that controls cell substitution for a call.
2. Description of the Prior Art
When carrying calls, telecommunication systems break down calls into packets, blocks, frames or cells depending on the protocol of the data network. The telecommunication systems lose some cells for various reasons such as problems with telecommunication devices. The telecommunication systems use cell substitution methods to compensate for these lost cells. Some examples of cell substitution methods are white noise, repeat last cell, and silence. White noise is typically background noise inserted because of users' expectation of some noise. Repeat last cell is another method of inserting cells based on a calculation of previous cells.
FIG. 1 depicts a system level block diagram of a prior solution for using a cell substitution method. In FIG. 1, a call processing system 120 is disclosed in a pending U.S. patent application Ser. No. 09/435,534, entitled “System and Method for Processing a Call,” filed on Nov. 5, 1999, which is hereby incorporated by reference. FIG. 2 depicts a message sequence chart of a prior solution for using a cell substitution method. A local telephone company 110 transmits an Initial Address Message (IAM) in Signaling System #7 (SS7) for a call to a signaling processor 122. The signaling processor 122 receives the IAM and processes the IAM. The signaling processor 122 then generates and transmits a route instruction for a routing system 124. Subsequent signaling messages for call set up are not shown for the sake of clarity.
The routing system 124 receives the route instruction from the signaling process 122. The routing system 124 uses one default cell substitution method that is provisioned by the user for all calls received from the local telephone company 110. The routing system 124 then receives the call from the local telephone company 110 over a call link 1112. The routing system 124 routes the call to an Asynchronous Transfer Mode (ATM) network 130 based on the route instruction. The routing system 124 uses the cell substitution method for lost or misinserted cells for the duration of the call.
One problem is that one cell substitution method does not provide the best performance for all different call types. The call types could be voice or data calls. Data calls are calls that carry data using modem, facsimile, or Switched 56. Different call types may use Signaling System #7, Integrated Services Digital Network, or Channel Associated Signaling for signaling and/or to carry the calls. Studies have shown that voice calls are optimized when white noise is inserted for lost or misinserted cells. Studies have also shown that data call performance improves when the repeat last cell method is used to substitute for lost or misinserted cells.
One cell substitution method for all calls results in diminished quality for certain call types that perform better with another cell substitution method. For example, data calls lose some data using a cell substitution method such as white noise that is optimal for voice calls. To improve call quality, telecommunication service providers need to utilize different cell substitution methods for different type of calls.